In motors, a short cylindrical stator core made of a magnetic material has a large number of concave slots and a large number of convex magnetic poles (circumferentially) alternately arranged at the inner periphery and magnet wires are wound around the magnetic poles and concurrently inserted in tiers in the slots, whereby a stator for generating a magnetic field is formed.
To efficiently obtain large rotational torque, the motors must have a high space factor of magnet wire (volume ratio of magnet wire) in each slot (space). Magnet wires are conventionally known that have a rectangular cross section in order to reduce the clearance between turns of the magnet wire placed in each slot (see, for example, Patent Document 1).    Patent Document 1: Published Japanese Patent Application No. 2005-174561